The tube can notably be an amplifier such as a travelling wave tube, having the French acronym TOP (Tube a Ondes Progressives) or the English acronym TWT (Travelling Wave Tube), or a klystron, for example. It can also be an oscillator (magnetron, etc. . . . ). Typically, it is desired to send the energy amplified inside the tube to a waveguide which contains air. The RF frequency window provides free passage, at least within a given frequency band, of the electromagnetic energy into the waveguide while maintaining the tightness to the vacuum inside the tube.
Conventionally, the windows comprise a flat disc made of an insulating dielectric, through which the electromagnetic energy passes. This disc is most often made of alumina or another ceramic having not only very good dielectric properties but also good thermal conductivity and good resistance to high temperatures and steep temperature gradients. In fact, for high power tubes operating with high electric fields, the passage of the energy gives rise to losses in the dielectric and therefore to significant heating. The tubes of concern here can supply powers of several tens of kilowatts. The dielectric disc can typically have dimensions of about ten centimeters in diameter for a thickness of between 1 millimeter and a few millimeters.
In order to obtain the vacuum-tightness, the dielectric disc is brazed around the whole of its periphery onto the inside surface of a cylindrical metal skirt (generally made of copper) which surrounds the dielectric disc.
From the European patent EP 1 364 425 EP (Thales) are known embodiments which make it possible either to carry out a shrink fitting and to apply a prestressing to the assembly or to chill without producing prestressing.